It is well known that air quality must be strictly maintained in a cleanroom where semiconductors are manufactured. Due to the extremely small geometries on state-of-the-art random access memories and other semiconductors, microscopic impurities such as fibers, dust, and pollen found in normal environmental air will, if released in a manufacturing cleanroom, render any product produced unusable. Even fibers shed from the worker's clothes and skin cells sloughed off the workers in a cleanroom would normally make semiconductors produced in the presence of these particulates inoperative. In addition to particulate concerns, the temperature and humidity of the air must be strictly controlled to maximize product yield. For example, if the relative humidity drops significantly, static electricity discharges would likely destroy circuit elements on the semiconductors. Temperature changes cause variations in certain processes involved in making semiconductors, as well as affecting relative humidity. For these reasons, the air in a cleanroom must be constantly recirculated, purified, and conditioned.
Cleanrooms have different degrees of cleanliness, ranging from Class zero to Class 100,000, depending on the number of particles one micron or larger that are found in each cubic foot of air. As a comparison, typical unfiltered air has about one million particles one micron or larger in each cubic foot of air.
Various types of equipment have been used in the past to help remove particulates from objects or from the worker either before or after they enter the cleanroom. An example is an invention by Byrnes described in U.S. Pat. No. 4,624,690. This apparatus is used to remove particulates from smaller objects such as the worker's gloves, the worker's boots, or from wipes or other articles both before and after they enter the cleanroom. Air showers are also commonly used which remove particles from the entire worker before s/he enters the cleanroom.
One component found in manufacturing cleanrooms which aids in removing particulates from the air is a number of recirculating fans. Recirculating fans move air rapidly from the cleanroom, where the product is manufactured, through humidity and temperature regulators, to air filters, where the accumulated particulates shed by the workers and the equipment are removed, and back to the cleanroom. The recirculating fans also serve to keep the cleanroom pressurized, which helps to keep particulates from entering the cleanroom. To date, the recirculating fans have caused certain problems of their own, not the least of which are maintainability and the space they require.
Early layouts of manufacturing cleanrooms had one large fan servicing a single manufacturing bay, or a number of bays. Advantages of this layout included ease of installation, low installation cost, and relatively low maintenance. As manufacturing technology improved, however, it became necessary to more strictly control environmental conditions such as particulate count, temperature, and humidity of the air in the cleanroom, because as geometries decreased these parameters more greatly affected product yield. One method which helps achieve this end is to increase the number of fans so that only a portion of a bay, or "zone" of the bay, is serviced by a fan. As geometries continue to decrease, even finer control of environmental conditions is needed, which requires more fans to keep cleanroom conditions within the desired limits. This has created a space problem, as the fans and associated service areas will be larger than the zone they are servicing. Space requirements in a state-of-the-art cleanroom are already no small consideration, as it has been estimated that for every square foot of cleanroom floor space, three square feet of space are required for support equipment.
Maintainability of the fans is also a problem, as in present designs of manufacturing cleanrooms the entire cleanroom must be shut down in order to service a single fan. With more and more fans, the chance of a failure, which would require a shutdown of the cleanroom, also increases. Adding a backup fan would help the maintainability problem, but would compound the space problem.